A multirod collimator for neutron therapy.

PURPOSE: To design, construct, and commission a multirod collimator for producing irregularly shaped fields in neutron radiation therapy. To demonstrate the reliability and applicability of this device to routine use with a superconducting cyclotron for neutron therapy. METHODS AND MATERIALS: A multirod collimator has been designed, constructed, and thoroughly tested to investigate its radiological properties; neutron transmission characteristics, beam profiles, and penumbral widths as a function of field size and depth in a phantom, and the spatial resolution of the rod array, have been measured. A wide variety of irregularly shaped fields, used routinely in neutron radiation therapy, have been produced, including fields that incorporate partial transmission blocks. The performance of the collimator has been closely monitored over a period of 20 months to accurately assess reliability. RESULTS: The multirod collimator has been in routine use for 32 months, and during this time a total of 7025 neutron fields has been treated. For the latter 20 months of this period, detailed performance records show that collimator failure has caused 28.4 h of downtime during the patient treatment day. Only 5.25 h of this downtime was experienced in the last 12 months (0.22% of the available treatment time). The results of collimator attenuation and beam profile measurements show that the radiological properties of the collimator are comparable to those of other collimator systems used for neutron radiation therapy. Isodose measurements in a water phantom show that the spatial resolution of the rods is superior to that of the leaves used in neutron multileaf collimators. The ability of the multirod collimator to produce many irregularly shaped fields commonly encountered in neutron radiation therapy has been demonstrated. Shaped fields for prostate, head and neck, soft tissue sarcomas, lung, thyroid, rectum, bladder, colon, breast, pancreas, and gynecological tumors have been produced. For some prostate cases, the device has been used to produce partial transmission blocks. CONCLUSIONS: A novel multirod collimator has been designed, constructed, and successfully applied in the routine treatment of neutron radiation therapy patients.

Cryogenic aspects of the operation of a superconducting cyclotron-based neutron therapy facility.

The Harper Hospital superconducting cyclotron, which is used for neutron radiation therapy, is a unique device. It is the first superconducting cyclotron to be installed in a hospital. The novel magnet cryostat can be rotated through 360 degrees without spilling liquid and whilst remaining vented to a low pressure return line for collection of the boil-off gas. The mode of operation of the cryogenic magnet is described in detail. Some of the problems associated with the cryogenic nature of the cyclotron including those problems encountered in operating a helium liquefaction system in a hospital are discussed. At the present time the magnet is kept cold by filling the cryostat with approximately 75 L of liquid helium each day before patient treatments begin. This is a time-consuming process. The possibility of modifying the helium gas recovery and liquefaction system so that a continuous liquid helium supply could be delivered to the magnet cryostat is discussed.

Calculations of x-ray and neutron transmission through multirod arrays.

Multirod arrays can be used to produce irregularly shaped irradiation fields for use in external beam photon and neutron radiation therapy. Two prototype multirod collimators have been built for use with high energy photon beams. A practical multirod collimator is in routine use with the fast neutron beam at a superconducting cyclotron based neutron therapy facility. A simple computer program has been written for calculating the transmission of photon and neutron beams through multirod arrays. The results of calculations for both close packed and spaced rod arrays are presented, and compared with available photon and neutron transmission data. The transmission through a regularly packed array exhibits a pattern of maxima and minima which occur with a spacing corresponding to the rod radius. The program predicts the positions and magnitudes of the transmission peaks. The rod diameter and spacing, the source size, and the position of the multirod collimator and the measurement plane relative to the source, all effect the exact nature of the measured transmission pattern. The transmission of 15-MV photons through close packed and spaced rod arrays was calculated using the program and compared with measurements made in a close packed tungsten rod array and with a prototype multirod collimator. Calculations for the transmission of a p(42)-Be neutron beam through a close packed tungsten rod array were compared with previously published data. Good agreement between calculations and measured data was obtained in all cases. The program was used to design a practical multirod collimator for a d(50)- Be fast neutron beam.(ABSTRACT TRUNCATED AT 250 WORDS)

Radiological properties of a prototype multi-rod collimator for producing irregular fields in photon radiation therapy.

A prototype multi-rod collimator for producing irregular fields in photon radiation therapy has been designed and built. The mechanical details of the design and operation of the multi-rod collimator are discussed. Beam profiles for an approximately 10 x 10 cm2 field have been measured at various depths in phantom, and compared with profiles obtained using the secondary collimator jaws alone and with cast metal blocks. The ability of the collimator to produce irregular fields is demonstrated with reference to some commonly encountered therapy fields and the ability to produce central blocks and island blocks is discussed. Isodose curves for selected irregular fields are presented.

A superconducting cyclotron for neutron radiation therapy.

The physical and clinical specifications of a neutron therapy facility utilizing a superconducting cyclotron are presented. The cyclotron and its support system are described. Details of the operation of the cyclotron in a hospital environment are given; the requirements of the helium liquifier and cryogenic system are described together with a summary of its mode of operation. The simplicity of the cyclotron control system is discussed. The physical characteristics of the neutron beam are described. The central axis percent depth dose curve is equivalent to that of a 4 MV x-ray beam. The depth of maximum dose occurs at approximately 9 mm depth and the surface dose is between 40% and 45%. The multirod collimator allows for the production of irregularly shaped fields of size up to 26.5 x 30 cm, without excessive exposure to operating personnel.

Transmission measurements in multi-rod arrays: a design study for a multi-rod collimator.

The results of transmission measurements for neutrons, cobalt-60 gamma-rays, and 10 and 15 MV photons made with close-packed arrays of tungsten rods are presented. These results indicate that tungsten rod arrays of reasonable thickness can provide for primary or secondary collimation of all these radiation beams. Development work on a collimation system utilizing the multi-rod concept which is capable of producing irregularly shaped fields and suitable for use in photon or neutron radiation therapy is described.